Test of the generalized Brink-Axel hypothesis in Ni 64,65

Previously published particle-γ coincidence data on the Ni64(p,p′γ)Ni64 and Ni64(d,pγ)Ni65 reactions were further analyzed to study the statistical properties of γ decay in Ni64,65. To do so, the γ decay to the quasicontinuum region and discrete low-lying states was investigated at γ-ray energies of 2.0-9.6 and 1.6-6.1 MeV in Ni64 and Ni65, respectively. In particular, the dependence of the γ-strength function with initial and final excitation energy was studied to test the validity of the generalized Brink-Axel hypothesis. Finally, the role of fluctuations in transition strengths was estimated as a function of γ-ray and excitation energy. The γ-strength function is consistent with the hypothesis of the independence of initial excitation energy, in accordance with the generalized Brink-Axel hypothesis. The results show that the γ decay to low-lying levels displays large fluctuations for low initial excitation energies.